1. Field of the Invention
This invention relates to a system for providing thermal insulation and, more particularly, to the use of a woven ceramic material impregnated with the ceramic matrix for providing enhanced durability.
2. Description of the Related Art
In the design of thermal protection systems for aerospace vehicles, such systems should not transfer excessive heat to the basic vehicle structure, should have low weight, and should produce low thermal stresses. Current thermal protection system concepts employ multitudes of ceramic tiles and flexible blankets as reusable surface insulation. For example, the U.S. Space Shuttle Program uses a thermal protection system, which includes a rigid ceramic insulation core with an unreinforced ceramic coating. Also, the Space Shuttle uses a flexible ceramic insulation core with a ceramic fabric exterior surface. Such tiles and blankets have the disadvantages of being subject to surface cracking and impact damage. In an effort to improve durability of the thermal protection system for the Space Shuttle Program, a toughened unipiece fibrous insulation, known as "TUFI" has been developed. TUFI comprises a ceramic solution with ceramic fillers, which are used to increase the density (hardness) of the exterior surface of the insulation core. However, Tufi is still subject to impact damage.
Another solution proposed and demonstrated by NASA is a ceramic matrix composite mechanically attached to a rigid insulator core. This thermal protection system is disclosed and claimed in U.S. Pat. No. 4,713,275, issued on Dec. 15, 1987. The attachment means include a self-locking wedge and complementary surface, ceramic pins, augers and the like.
Mechanical attachment has a primary disadvantage in that the ceramic core insulation has low resistance to fatigue loads.
Another thermal protection system used in the Space Shuttle Program is a flexible, thermal insulation blanket, which uses a flexible insulator core formed of ceramic materials sandwiched between two ceramic fabric layers and joined with ceramic thread. The sewing threads create a pillowing condition between the threads resulting in a non-uniform, unsmooth exterior surface. This causes undesired friction between adjacent fibers during flight and potential failure. Furthermore, the non-uniform, unsmooth exterior surface precludes the use of this material on the lower surface of the Space Shuttle because its use on this area would create unacceptable boundary layer conditions.